1. Field of the Invention
The present invention relates to a communication apparatus utilizing a public line such as a facsimile apparatus, a telephone set, and a personal computer.
2. Description of the Related Art
FIG. 14 is a circuit diagram showing a line interface unit (LIU) circuit 41 of a conventional facsimile apparatus. The LIU circuit 41 is connected to a telephone line 43 via connection jacks 48a, 48b. A telephone set 42 is connected to the telephone line 43 in parallel with the facsimile apparatus. Further, the LIU circuit 41 is connected to an external telephone 51 via connection jacks 50a, 50b.
A transformer 44 is connected to a change-over switch 45 which changes the connection of the telephone line 43 to a modem side of the facsimile apparatus or to the external telephone 51 side. A hook detector 46 is connected between the change-over switch 45 and the connection jack 48a and an attester 47 is connected among the connection jacks 48a, 48b and the change-over switch 45 in parallel with the changeover switch 45 in order to protect a facsimile apparatus by absorbing a surge voltage which might be generated on the telephone line 43.
The hook detector 46 comprises a photocoupler PC2, a resister R13, a condenser C12, a varister VA1, and a resister R14 being connected between the transformer 44 and the resistor R13. The phtoeoupler PC2 comprises a phototransistor and a light emitting diode which are connected in parallel with each other and arranged in a direction contrary to each other, and electrical continuity of the phototransistor is obtained by emission of the light emitting diode caused by an input signal. In addition, a resister R15 is connected between the transformer 44 and the change-over switch 45. The resistors R13, R14, R15, the condenser C12, and the varister VA1 regulate frequency characteristics and signal levels from the facsimile apparatus.
A hook detector 49 is connected between the change-over switch 45 and the connection jack 50b. The hook detector 49 comprises a condenser C11, a resister R11, a photocoupler PC1 which are connected in parallel with each other, and a resister R12 being connected between the resister R11 and the photocoupler PC1. The operation of the photocoupler PC1 is the same as that of the photocoupler PC2, and the condenser C11 and the resistors E11, R21 regulate frequency characteristics and signal levels from the external telephone.
For example, when a telephone set with or without a recording function, facsimile apparatus, or personal computer is used in a house or smaller office, these apparatus are not independently connected to a line dedicated for each of these apparatus. Instead they usually share one public line. When the telephone 42 and a facsimile apparatus (only the LIU circuit 41 shown) are connected to the telephone line 43 in parallel as shown in FIG. 14 and the facsimile apparatus is arranged apart from the telephone 42, the facsimile cannot detect whether the telephone 42 is busy unless a call or other transmission occurs via the facsimile apparatus.
The facsimile apparatus lacks a circuit configuration capable of detecting a state of a closed direct current (DC) circuit (off hook state) and an open DC circuit (on hook state). In other words, this hook detector 46 can only detect the hook state of the facsimile apparatus, and the hook detector 49 can only detect the hook state of the external telephone 51. The problem is that the facsimile can not determine whether the telephone 42 is busy unless a call or transmission occurs via the facsimile apparatus.